Particularly for traffic control technology, a system is provided in which data transmission is done from a vehicle moving past a stationary beacon to that beacon. Since it is not practical to equip the vehicles themselves with a transmitter, provision is made for having carrier signals transmitted continuously from the beacon; these signals are then received by the vehicle moving past and are modulated with a data signal. The carrier frequencies in question are in the microwave range (such as 5.8 GHz). The carrier signal is received by the antenna of the vehicle moving past and is modulated with a data signal generated locally in the vehicle.
It is known for the modulation to be done at the base of the antenna at different frequencies provided for the two logical states of the data signal and for the carrier signal to be reflected in modulated form from the antenna, so that transmission power of its own need not be furnished by the vehicle. The modulation methods used are typically frequency-encoding ones, by which the carrier frequency is shifted by a different frequency as a function of logical "1" than as a function of logical "0" (this is known as frequency shift keying). To enable furnishing the on-board unit as mass-produced equipment with the least possible expense, the attempt has been made to design suitable encoders as simply as possible.